The Romans made of Europe, Asia, and Africa a vast empire ; the weakness of the peoples and the tyranny of command united all the parts of this immense body. At that point the Roman policy was to separate itself from all nations that had not been subjugated : for fear of bringing to them the art of conquering, they neglected the art of enriching themselves. They made laws to prevent any commerce with barbarians. “Let no one,” say Valens and Gratian, “send wine, oil or other liquors to the barbarians, even to taste them”  ; “let no one take gold to them,” add Gratian, Valentinian and Theodosius, “and even what they have of it should be slyly made off with.”  The shipping of iron was forbidden on pain of death.
Domitian, a timid prince, had the vines pulled up in Gaul,  for fear, no doubt, lest that liquor attract barbarians there. Probus and Julian, who never feared them, had the vines replanted.
I quite realize that, in the weakness of the empire, the barbarians obliged the Romans to establish market places  and to trade with them. But even that proves that the spirit of the Romans was not to trade.